Disputes over payment for the Bushehr plant, which Tehran says will be the centre piece of a civilian nuclear power program, have led to increasing indications that Moscow is putting the brakes on the Iranian project.
"We are now in a crisis situation, and the current stage of talks is aimed at finding a way out of it," Irina Yesipova, spokeswoman for state nuclear power plant builder Atomstroiexport, told Agencie France Presse.
Yesipova said it was "quite likely" the plant would not be operational until late 2008 because of underpayment by Iran, adding: "It is obvious that Iran has broken the trust of our subcontractors" by missing payments.
Iranian nuclear negotiators Javad Vaeidi and Mohammad Saeedi flew to Moscow from Vienna late on Tuesday for talks aimed at persuading Russia to complete the project.
Russia has repeatedly rejected Iranian claims that the Bushehr plant could be launched this year – a position welcomed in Washington, which has urged Moscow to delay the launch.
The Bushehr power plant is considered proliferation safe, but Washington fears that Iran’s civilian nuclear programme could be a smoke screen for nuclear weapons development.
Earlier on Wednesday, a Russian subcontractor working on Bushehr called a 2007 start date "unrealistic."
"Even if Russia delivered the first shipment of nuclear fuel to Bushehr tomorrow, it would be impossible to open the station within six months," Russian news agency RIA Novosti quoted Energoprogress director Ivan Istomin as saying.
"The real date for the physical launch of the power plant is being delayed until autumn 2008," he said.
Construction stalled earlier this year over charges that Iran had fallen behind on payments, as well as delays in shipping parts for the plant from third countries.
"Payments are coming now, but not in the amount we need. This has affected our subcontractors’ financial situation," Atomstroiexport spokeswoman Yesipova said.
Iran has been hit with two rounds of UN sanctions for failing to suspend enrichment of uranium, and is threatened with a third round for continuing enrichment.
The Bushehr nuclear power plant project, which was first contracted to a German nuclear builder in 1975, is now valued at $1.2 billion.